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Fraser
2004-Nov-12, 04:44 PM
SUMMARY: This photograph of Phobos, one of Mars' two tiny moons, was taken by the European Space Agency's Mars Express spacecraft when it was less than 200 km (125 miles) away during a recent flyby. The picture shows the strange parallel grooves that run around moon, and researchers might be able to tell whether they formed before or after the larger impact craters. Phobos is locked in a "death spiral" around Mars, and it'll eventually crash into the planet, or be torn apart and turned into a short-lived ring.

What do you think about this story? Post your comments below.

antoniseb
2004-Nov-12, 04:56 PM
These are great images. It's great that the ESA probe gets so close to Phobos on a regular basis. I wonder what caused the rings partway into the deeper craters.

scott
2004-Nov-14, 12:40 AM
I wonder if the lines are cracks causes from either gravitational interaction or some near terminal impact?

Janice
2004-Nov-15, 12:55 AM
:D Cool Picture :D

wstevenbrown
2004-Nov-15, 01:09 AM
Sedimentary, my dear Watson. I take the striations as evidence that Phobos is a fragment of a much larger body, which formed by adhesion/cohesion of dusts and ices, probably in trans-Plutonian space. Close observation of the striae should at least fix a lower limit to how large the parent body was, based on their radius of curvature. I say 'probably', thinking the while that either half of a spall could have ended up in low Mars orbit. Intuition sez it's a fragment of the impactee. The only sure thing is, it didn't form at or even near Mars. These could be the tree rings that allow us to track the local weather for the last few gigayears, given a known keying event that can be read. Reactions? :rolleyes: Steve

Don Dowden
2004-Nov-15, 04:53 PM
What causes the death spiral? Mars has a thin atmosphere, but the moon is presumably above the Martain atmosphere in any case? A drag would seem to be necessary, but what is the source of the drag?

antoniseb
2004-Nov-15, 05:00 PM
Originally posted by Don Dowden@Nov 15 2004, 04:53 PM
What causes the death spiral?
Things in a faster than synchronous orbit are pulled in by tidal forces, while things further than synchronous orbit [such as our moon] are propelled out and eventually escape.

As to Steven's idea that it was formed in trans-plutonian space, allow me to point out that this thing is in a low circular equitorial orbit around Mars. How did a dead comet get there?

VanderL
2004-Nov-15, 05:46 PM
Sedimentary, my dear Watson.

Not quite dear Sherlock, some of the striations (I think most of them) are smaller and larger "Crater Chains", for lack of a better word. Crater chains are thought to happen as a result of fragmenting impactors, on Mars itself they are thought to be caused by subsurface erosion (mostly from volcanic activity), and are called "collapse pits".
Crater chains can be found all over the solar system (Ganymede, Callisto, Io, the Moon, Phobos, Mars, a few on Earth to name some), some people think they were created in an battle of ET's (haha, plasma torpedoes away!).
I think (no surprise here) that the crater chains are evidence of electric machining; giant discharges that strip the surface raw and leave very specific "fingerprints".
It could mean that all the bodies in the solar system showing crater chains were subjected to these electrical forces, in case of Phobos, maybe the same event that destroyed the Martian surface and atmosphere shot Phobos into it's strange orbit, only to return to Mars' surface eventually?


Cheers.

spacepunk
2004-Nov-16, 04:00 AM
Excellent picture. The striations have the appearance of sedimentary rock -- is this unique for asteroids studied to date? Questions do arise as to it being an object blasted off of Mars surface (where there were sedimentary rock layers) from a meteor impact, or from another planet that had sedimentary rocks. Trans-plutonian? A remnant of the failed planet in the asteroid belt? Or perhaps it's from an external solar system much older than our own and long since gone from the skies ... it would be useful to get a radiometric date and mineral/element composition on this captured asteroid/moon of Mars. One would then see if it has unique age and chemical properties in addition to its appearance.

As for its death spiral, it would be a useful endeavor to figure out a way to save this asteroid from crashing into Mars. It wouldn't have to be a permanent repair on its orbit, rather do much like the leaning tower of Pisa where just enough propulsive force is used to put it in a safer orbit than it is now. It would be less costly and more practical than putting it into a permanently safe orbit because it would be a tourist attraction for the next few generations of earthlings (as it is for us right now). Who knows, the spinoffs of devising such a method to move a massive object such as Phobos into a higher orbit could help us deal with near-Earth-asteroids (NEO's) if one came too close to Earth in the future.

spacepunk
2004-Nov-17, 01:57 AM
Here is a newstory from this summer about a planned attempt to move an asteroid. Basic physics and practical learned would be usefull for moving Phobos temporarily out of its death spiral. Admittedly this would be slightly more complicated the regular repositioning of the ISS and other artificial satellites into a higher orbit above Earth

http://www.universetoday.com/am/publish/ne...id.html?1472004 (http://www.universetoday.com/am/publish/new_plan_move_asteroid.html?1472004)
http://www.universetoday.com/forum/index.p...t=ST&f=2&t=3953 (http://www.universetoday.com/forum/index.php?act=ST&f=2&t=3953)

wstevenbrown
2004-Nov-17, 02:21 AM
My dear VanderL-- I would buy the crater chains explanation, if there were not so many of them, and if they were not all near-as-da**it parallel. As to how a foreign object got into low Mars orbit-- like the Captain who found a coelacanth on his deck, I don't know.
The notion that it might be a spall off Mars itself has merit-- then the striae would be more than metaphorically sedimentary. Achieving the required near-circular orbit is problematical-- a spall starts out with one focus of its ellipse at planetary surface--take a long time to make the foci coincide, eh?
Let's go look! Regards--Steve

VanderL
2004-Nov-18, 06:55 AM
My dear VanderL-- I would buy the crater chains explanation, if there were not so many of them, and if they were not all near-as-da**it parallel

Hi Steven, the number of crater chains is just an indication of the amount of discharges and the characteristsics of the material being "blasted" by the discharge(s). Here (http://www.thunderbolts.info/tpod/2004/arch/00current.htm) is an example on the Moon where a sinuous "crater chain" cuts through a montain range, as stated in the text, overlapping craters form into channels. The same features can be found on Mars, the largest features (Valles Marineris) could very well be the place where Phobos was ejected (discharges are very good at cleanly removing material just like in industrial electro-dynamical machining or EDM).

These crater chains and channels can be found on almost every Moon or rocky planet. I know it sound crazy, but this mechanism could explain features like channel running uphill.


Cheers.

JESMKS
2004-Nov-19, 05:24 PM
The same photo is on APOD today. The striatations sure look like layers in a sedimentary sequenc. Maybe the asteroid belt was created by the destruction of a planet containing sedimentary rocks. The explosion that caused it's destruction sent a fragment towards Mars where it was captured. If fragments of Mars can be found in Antartica, Phobos could be from a destroyed planet.
Jack

wstevenbrown
2004-Nov-19, 07:34 PM
If they are truly sedimentary, odds are Phobos was spalled from Mars. Didja notice how neatly I reversed my field, filed off the serial numbers, and embraced Spacepunk's idea as if it were my very own? My original thought was that the layers represent the accretion history of some medium-sized asteroid from out in the frigid zone, broken off by impact. In either case, its present orbit is difficult to account for. S

Duane
2004-Nov-19, 11:43 PM
Well, there are a few issues in this thread :)


Is Phobos a chip off a larger body? It seems likely, although not a body as large as a planet, and most assuredly not from Mars. Why not Mars? Because the moon has been studied slosely, including its spectrum, and it composition does not look at all like Mars. Instead, its composition is the same as main-belt asteroids.

As for moving Phobos into a higher orbit, what's the point of doing anything for the next several hundred thousand years? It's orbit is decaying, but it won't actually collide with the planet for several million years.


the number of crater chains is just an indication of the amount of discharges and the characteristsics of the material being "blasted" by the discharge(s).

There is no evidence that an electrical charge of any size can "blast material" leaving a string of well formed craters in a perfect line. If anything, this makes the idea of a broken up impactor much stronger.

More of the "it looks funny so there is something wrong with our theories" baloney.


(discharges are very good at cleanly removing material just like in industrial electro-dynamical machining or EDM).


A completely different thing. Like comparing apples to t-bone steaks.


maybe the same event that destroyed the Martian surface and atmosphere shot Phobos into it's strange orbit, only to return to Mars' surface eventually?


Well again, Phobos' composition is nothing like Mars', so it would be very strange to see two completely different objects arising from one event. Further, the surface of Mars is not "destroyed", it's different. There are several epoches in Martian history, and, again, it does not require any "exotic" explanation for any of them.

VanderL
2004-Nov-20, 12:06 AM
There is no evidence that an electrical charge of any size can "blast material" leaving a string of well formed craters in a perfect line.

Well, you can se the evidence right in the pictures, there is no evidence that crater chains are impact craters, a whole host of those chains are called "collapse pits" on Mars. They can be straight as well as curved, depends on how the discharge was created and on the conductivity/structure of the surface being blasted.


QUOTE
(discharges are very good at cleanly removing material just like in industrial electro-dynamical machining or EDM).


A completely different thing. Like comparing apples to t-bone steaks.


Why is that, it is exactly the same process, electric discharges can be scaled up.


More of the "it looks funny so there is something wrong with our theories" baloney.

No, it just looks like electric discharges, so what we see in the images confirms large-scale electric discharges.

I didn't know the composition of Phobos was different from Mars, do you have any references?
Thanks,

Cheers.

VanderL
2004-Nov-20, 04:52 PM
Duane:
Further, the surface of Mars is not "destroyed", it's different. There are several epoches in Martian history, and, again, it does not require any "exotic" explanation for any of them.


Mars' surface is definitely destroyed, did you notice that there is a strong dichotomy between the Northern and Southern hemisphere? Thatís a whole planet that shows evidence of a catastrophe, or do you think this difference is a logical consequence of planet formation? No tectonics, and still the surface shows extensive "remodelling" to use a euphemism. Science tends to use slow processes to explain them. Catastrophic events do happen, and they did happen to Mars.
The different epochs you stated are all based on the "crater counts" method. This method is completely hypothetical and whatís more, if the electric explanation holds water, it is completely wrong as well.
The explanation is "exotic" because it is new, but there are certainly enough indications that electric forces play important roles in the history of the solar system (and even the Universe at large).
Dismissing this explanation because it isnít necessary, is closing your eyes to novel insights. You can disagree with the ideas, but that doesnít mean they are wrong.

For those interested Iíll give a link to the late
Ralph Juergens' (http://www.kronia.com/library/electrical1.html) ideas (around 1970) about the scarring of the Moon and Mars. He was heavily influenced by Velikovskyís work, so if you have a problem with that, just read the parts about the evidence for electrical scarring of the Moon and Mars.
I hope it shows how these features are interpreted as evidence of electric discharges, and I hope it shows what I mean when I say that crater chains are evidence of electric scarring.
It is a long piece (2 parts) and the images are of course very old, for close-up images of Mars, there is a series of "collapse pits" shown on www.spaceref.com from the THEMIS archives.
The Thunderbolts.info website also shows images of electric scarring on the Picture of the Day archive.
For what it's worth, I think these people are on to something important and Duane, I'm aware that their ideas need much more testing, and your constant reminder that it is only "image picking" is noticed, but it only means that more work needs to be done. However your argument that all is already explained adequately and doesn't need revision is much too early to call.
Sure, it it will only be replaced by new explanations when the evidence is compelling. In my view the electric explanation deserves to be pursued, whatever the outcome. Don't dismiss it just yet.

Cheers.

wstevenbrown
2004-Nov-22, 03:10 PM
Steady, Duane. Remember that the "overall composition" of Mars is a statistical average over the area of an entire planet. Statisticians are the folks who will tell you that if you have one foot in boiling water, and the other encased in dry ice, you are perfectly comfortable, on average! To completely dispel the notion that Phobos is a spall off Mars, we would have to show that there is no possible location on the past or present surface from which it could have come-- after all it only came from one location, wherever that might have been, and however atypical of the surface as a whole. Rather than going through such an exhaustive "proof", my suggestion would be to actually assay the surface of Phobos. Isotopic abundances should tell us, by the way they vary with depth, what part of the solar system it came from, and how long it's been where it is.

VanderL, I am a strong proponent of the notion that EMF is the redheaded stepchild of the forces, as far as galaxy and star evolution are concerned. It gets neglected because it requires more initial conditions as a prerequisite to being able to calculate its effect. With gravity, if you see an object, you may safely assume that it is massive and that gravity is the prime mover in its celestial motion. That argument does not apply to atom- and molecule-sized objects. It applies very little to dust grains and rocks. At the scale of small asteroids, one can finally find that gravity is the dominant force, but for real precision, one should include EMF as a perturbation (websearch keywords: "Yarkovsky effect"). Problem is, to accurately calculate the effect requires a huge amount of knowledge of the object; e.g., composition, albedo variations, surface area presented to the illuminating body by an irregular multiaxis-rotating object, and so forth. The kind assessment of the EU thesis, tho, is that they paint with too broad a brush. The website is one long, shrill, self-defensive lament that puts me off ever discovering whatever insights they may have to offer. If they could get off the anti-us conspiracy theme, 'twould be much easier to hear what they're actually saying.
Best regards-- Steve
PS-- I wanna go to Phobos, pleeeeeeeze!

VanderL
2004-Nov-22, 03:57 PM
Hi Steven,


VanderL, I am a strong proponent of the notion that EMF is the redheaded stepchild of the forces, as far as galaxy and star evolution are concerned. It gets neglected because it requires more initial conditions as a prerequisite to being able to calculate its effect.

That's exactly the problem with assigning any importance to electrical effects anywhere in space, it is hard to "capture" in equations. The underlying assumption that hampers the EU view is the flawed notion that space is electrically neutral. There's enough charge around to power anything you like, if only it was realised that a plasma is not the perfect conductor where charge separation is impossible due to electrons quickly filling the gaps.

In case of Phobos, the crater chains visible on it's surface are caused by electric discharges (electric "etching"), just look at the way they follow the topography of Phobos. "Sticky", the large crater is sporting a crater chain over it's edge up and down, without any change in depth or direction, you can see the crater chains lining up (almost never in an exact straight line), and when the numbers of craters increase, they form overlaps and that's when they start to look exactly like the channels on Mars; sinuous with scalloped edges and flat floors.

If Phobos is not a piece of Mars, then it got there from some other source, it could even be a comet or asteroid captured by Mars.



The kind assessment of the EU thesis, tho, is that they paint with too broad a brush. The website is one long, shrill, self-defensive lament that puts me off ever discovering whatever insights they may have to offer. If they could get off the anti-us conspiracy theme, 'twould be much easier to hear what they're actually saying.

Sorry to hear this, it didn't put me off much. I can understand it from their point of view (exasperation, I guess), but if it puts you off, they're obviously on the wrong tack. I happen to know they are serious in their effort to get the EU model acknowledged.

Cheers.

Duane
2004-Nov-22, 06:56 PM
Well, you can se the evidence right in the pictures, there is no evidence that crater chains are impact craters, a whole host of those chains are called "collapse pits" on Mars. They can be straight as well as curved, depends on how the discharge was created and on the conductivity/structure of the surface being blasted.


Really VanderL? What evidence to you see? Collapse pits and crater chains are different, formed by two different processes. There are no pits caused by discharge anywhere on any body that has been closely looked at. The EU model of the u8niverse has been looked at by some pretty knowledgable people, most of whom have found it to be wanting from both a physics and observational point of view. It was an interesting therory that died when it was put up against hard evidence.

While there is some properties of electro-magnetism that are poorly understood and require more research, the EU model is not one of them.


Why is that, it is exactly the same process, electric discharges can be scaled up.


There is nothing to suggest that electrical discharges can be "scaled up" to the size necessary to create thousand mile long features on a planet. They are not the same process, not by far VanderL. Further, there is no evidence of charge buildup anywhere in the sun, the solar system, or the universe for that matter. Pray tell VanderL, where does this mystery electrical charge come from? It is laughable that the EU website points to a 3 meter long discharge scar, and then says "see here is eveidence that the 1000 mile long features on Mars form the same way".


I didn't know the composition of Phobos was different from Mars, do you have any references?


Put "Phobos" into a search engine, then read.


Remember that the "overall composition" of Mars is a statistical average over the area of an entire planet.

Well sure steven, except that the Martian composition has been precisely mapped to the level of a few meters. There is no where on Mars that is similar to Phobos. On the other hand, several hundred asteroids have the same chemical makeup of Phobos. Seems to me that some degree of common sense has to come into play. This also answers this quote:


we would have to show that there is no possible location on the past or present surface from which it could have come

which to me is backwards. Rather than showing there is no possible location it could have come from, you would have to show some where on Mars where it could have arisen. Certainly Valles Marineris would not be the location--it is completely different compositionally, and there is nothing about the feature to suggest it arose by anything other than naturally. This is another feature that can be explained by geology, notwithstanding the shrill cries of the EU people that it looks funny.


There's enough charge around to power anything you like, if only it was realised that a plasma is not the perfect conductor where charge separation is impossible due to electrons quickly filling the gaps.


Except that it has been shown that this is wrong. Tim Thompson explained it to you in the EU thread--which is where this discussion belongs.


In case of Phobos, the crater chains visible on it's surface are caused by electric discharges (electric "etching"),

No VanderL, they do not look at all like something that could arise from a big lightning strike. They look exactly like what you would expect from an impact on a small rocky object.

I agree with Steven's assessment of the EU website--in fact, it seem to characterise the EU people as a whole. If they had a good, working theory with solid evidence and physics behind them, they would not need to be so shrill. The problem is that they don't! So, instead of dealing with the physics and observations, they retreat into the shrill cries of "we are being persecuted".


Mars' surface is definitely destroyed, did you notice that there is a strong dichotomy between the Northern and Southern hemisphere? Thatís a whole planet that shows evidence of a catastrophe, or do you think this difference is a logical consequence of planet formation? No tectonics, and still the surface shows extensive "remodelling" to use a euphemism. Science tends to use slow processes to explain them. Catastrophic events do happen, and they did happen to Mars.


Frankly, this is an example of taking the "it looks funny" dictum to an absurd level. The planet does not show evidence of a "catastrophe" it shows evidence of 4.6 billion years of life as a planet. I have gone over the geology of Mars with you once before VanderL, or have you forgotten that? As a refresher, go to this link:

http://www.universetoday.com/forum/index.p...topic=2150&st=0 (http://www.universetoday.com/forum/index.php?showtopic=2150&st=0)


However your argument that all is already explained adequately and doesn't need revision is much too early to call.

I have never said it is already explained or that there is no need for further work to revise our understanding. WHat I have consistantly said is that our understanding of the causes of the features seen on this planet and others in the solar system are cleaner, simpler and easily understood, whereas the EU explanation is an other-worldly, complicated, unseen cause that cannot even meet the laws of physics. The shrill yelling from the EU crowd does not change that simple statement.

antoniseb
2004-Nov-22, 07:07 PM
Can we move this discussion to the EU Thread please?

The images of Phobos are what they are, and they are pretty cool. But the EU tangent this thread is taking is robbing the loyal EU thread readers of some good insights as to what is going on here. It is also having an impact on those just interested in the image of Phobos. As you know we are making a similar effort to keep another 'Alternative Theory' confined to its thread only, and this dialog really suffers from the same problems.

Thanks in advance.

wstevenbrown
2004-Nov-22, 07:53 PM
the Martian composition has been precisely mapped to the level of a few meters. There is no where on Mars that is similar to Phobos.

I'm a little surprised to learn this, as Earth hasn't been mapped to that precision, other than by interpolation. Too, for an unknown period of time, Mars has been weathering and redepositing, and Phobos has been space weathering. Bear in mind that I am devil's advocate for this viewpoint-- I just don't think it has been eliminated from consideration.
My first impression (see earlier entries in this string) remains that Phobos is a spall off an object that accreted much further out, Vesta-size or larger, judging by the lack of discernible curvature in the striae. I'm utterly certain nothing could accrete in that pattern in that orbit-- but how did it get there? Food for thought: a near-miss trajectory resulting in aerobraking into low orbit-- resultant heating causes outgassing/melting/bubbling along natural fault lines (boundaries of the accretion layers). :huh: Singular events don't have to be likely, they just have to be possible. Regards-- Steve

antoniseb
2004-Nov-22, 08:01 PM
Originally posted by wstevenbrown@Nov 22 2004, 07:53 PM
I'm a little surprised to learn this, as Earth hasn't been mapped to that precision
There are fewer square meters on Phobos than on Earth, and no intervening atmosphere. This sort of mapping shouldn't be too hard for such an object.

Duane
2004-Nov-22, 08:05 PM
I'm a little surprised to learn this, as Earth hasn't been mapped to that precision,

Ha, that is absolutely true! We have better details of Martian composition than we do of Earth--or a least until recently. Seems to me they did some precise mapping just in the last year or so. I'll have to see if I can find it.

I agree with the remainder of what you say. My only point is that I don't see that Phobos was part of Mars at any point. Aerobraking--yea, that is a good theory Steven, as good as any other I've seen.

Darryl Biech
2004-Nov-23, 02:33 AM
Since it supposedly will take some 50 million years for the orbit of Phobos to decay
no point in worrying about that (not for millions of years anyway). I have read
some evidence that there is water ice under the surface. Since the sunny side
is 25 F, You could reflect enough light from the walls of this crater RIGHT at the space station to heat the area to 70 F or so from 25 F.

WIth the "room temperature" ice would melt and be captured in the best way possible.

At the space station you could dissociate via electricty the water into hydrogen and oxygen if needed for fuel or air. Since air is 21% Oxygen and 78% Nitrogen and
less than 1% Argon we'd just need Oxygen and Nitrogen brought along in large amounts obviously and later on just Nitrogen brought in if enough water ice is found for air (oxygen) and "space farming" etc! Perhaps a small hyrogen ION engine (Hydrogen dissociated from water used to fuel it) would be enuf to stop the small amount of orbital decay as well.

Soil could be analyzed to see what could be grown say on the floor of this
crater (ideally a space greenhouse). Being in that crater ought to provide some protection from meteorites as well.

Stickney the "Mother Crater" perhaps over decades could be turned into a "breadbasket". The extremely low gravity and proximity to Mars and the Asteroid belt make this a great place to move from for future space exploration.

CO2 from MARS could be stockpiled on the "cold side" of PHOBOS as "dry ice".
The temp on the dark side is said to be -170 F.

Electricity could also be generated on temp diff between the 25 F and -170F sides
if several miles of thin cable can be run.

Fraser
2004-Nov-23, 07:46 AM
I think you're talking about the SRTM mission that was flown on the space shuttle a few years ago. It mapped 80% of the Earth to a high precision. Here's a link:

http://www2.jpl.nasa.gov/srtm/

wstevenbrown
2004-Nov-23, 07:13 PM
Afterthought: The aerobraking hypothesis is easily testable, If true, Phobos will be 'cooked' and chondritic on the outside, to a depth of not more than 3m probably less), and 'raw' on the inside-- typical accretion stuff, tektites, dust, ice, organics, etc. The funny thing is, the aerobraking could apply both to an outsystem spall and to a spall off Mars. It renders the orbits non-Newtonian, and tends to circularize them. While such a singular event is wildly unlikely, it at least has the virtue of economy-- only one wild supposition to explain several bizarre facts. S

iantresman
2004-Nov-24, 12:39 PM
Originally posted by Duane@Nov 22 2004, 06:56 PM
There is nothing to suggest that electrical discharges can be "scaled up" to the size necessary to create thousand mile long features on a planet.
Plasmas and their characteristics are scaleable through at least 12 orders of magnitude (confirmed in the laboratory), and are probably scaleable by at least another 12 orders of magnitude; The similarities between a tiny spark and lightning are quite apparent.

There are megawatt currents flowing through space (see Electric Currents and Transmission Lines in Space (http://public.lanl.gov/alp/plasma/elec_currents.html))

Some craters, rilles and "scars" share characteristics with pits and craters created by Electrical Discharge Machining (EDM), and cathode arcs.

This does not prove that any crater or planetary features was formed this way, but it is evidence, and evidence that can be reproduced in the laboratory.

Regards,
Ian Tresman

iantresman
2004-Nov-24, 12:48 PM
Originally posted by Duane@Nov 22 2004, 06:56 PM
Further, there is no evidence of charge buildup anywhere in the sun, the solar system, or the universe for that matter. Pray tell VanderL, where does this mystery electrical charge come from?
Aren't all spacecraft subject to electric charging? Isn't the Earth charged, resulting in sprites, blue jets and elves discharging into space?

Regards,
Ian Tresman

antoniseb
2004-Nov-24, 04:16 PM
Friendly Reminder
Please keep the EU talk in the EU thread. Link to this story if you need to, but this thread has gone off on a tangent and is no longer discussing the close-up image of Phobos. You do a disservice to the readers of the EU thread to hide these insights here.

Thanks in advance.

TheThorn
2004-Nov-24, 11:38 PM
Originally posted by wstevenbrown@Nov 23 2004, 07:13 PM
Afterthought: The aerobraking hypothesis is easily testable, If true, Phobos will be 'cooked' and chondritic on the outside, to a depth of not more than 3m probably less), and 'raw' on the inside-- typical accretion stuff, tektites, dust, ice, organics, etc. The funny thing is, the aerobraking could apply both to an outsystem spall and to a spall off Mars. It renders the orbits non-Newtonian, and tends to circularize them. While such a singular event is wildly unlikely, it at least has the virtue of economy-- only one wild supposition to explain several bizarre facts. S
Aerobraking tends to circularize orbits, true. But doesn't it tend to circularize them by lowering the apoapsis (Is that the right term in the case of Mars?)? I can't see any way that it woudl circularize barely-captured asteroid's orbit by first loweing the apoapsis to an appropriate level, and then raising the periapsis to make it circular. It just doesn't work that way. Something else is needed to raise the periapsis.

Our orbiters that used aerobraking at Mars used a burn to lift periapsis (and end aerobraking).

antoniseb
2004-Nov-25, 12:01 AM
Originally posted by TheThorn@Nov 24 2004, 11:38 PM
Something else is needed to raise the periapsis.
I'm not yet a believer in this aerobraking idea for Phobos, but if something the size and mass of Phobos were in highly elliptical orbit, it would experience weak tidal forces that would slowly lower the maximum distance from Mars and raise the periapsis to a lesser degree.

Duane
2004-Nov-25, 04:04 AM
Good points, all. The biggest difference that I see about these arguments is that you are trying to compare the mass of a probe against the mass of an asteroid.

If the asteroid came into the Martian system at an angle that caused it to brake because of the atmosheric drag, it would not be a big step for it to become captured by the Martian gravity field. It doesn't require that the asteroid be subjected to "heavy" drag on each succesive pass, rather it is enough to say that the first past slowed it enough to be captured. This may also argue against any clear evidence of "burn" on the asteroid.

Newtonian physics will cause the orbit to circularize over time. If the asteroid has been subjected to the Martian grvity well for long enough, the orbit we see for Phobos is not unexpected.

wstevenbrown
2004-Nov-26, 08:52 PM
Good thinking, gents. 1st pass leads to capture, due to sufficient atmosphere penetration to result in drag. Apoapsis is reduced from an ideal point (indefinitely far out) to something more manageable. Periods longer than about 6mo. give such a long ellipse that perturbations by Earth and Jupiter result in chaos. I suggest that the second pass did not penetrate, but resulted in a stone-skip, a bounce, if you will. Admittedly, when I elect to skip a stone, I use a flat, round one for predictable results. What are the aerodynamics of a potato? perhaps we should consult NASA? In any event, a bounce would conveniently raise the periapsis so that Newtonian dynamics could reassert control.
Having multiplied two plausible-but-unlikelies together, I am starting to feel like a Velikovskyite. Is there an ointment for that?
I would very much prefer to see more, rather than 'explain' what we presently see. It's a lifestyle. Patience will bring in data presently beyond our grasp; meanwhile, we speculate. Frustrating, tho! Best regards, :rolleyes: Steve